1. The Field of the Invention
The present invention relates to compositions used in dentistry for sealing and/or filling a dental preparation. In particular, embodiments of the present invention relate to sealing and filling compositions that release a gas internally to offset the effects of shrinkage that occurs during polymerization.
2. The Relevant Technology
Resins are frequently used in dentistry and other fields as sealers and fillers. Recent developments in resin technologies have produced very strong resins with superior bonding properties.
Resins are useful sealants because they can be made non porous with strong bonding characteristics. The ability to resist permeation prevents bacteria and other foreign matter from contaminating the underlying tissue.
Resins are good fillers because they are easily shaped, bond well, and are very hard and durable. Resins are typically fluid when applied to a material. Because the resin is initially fluid, it can take the shape of a void, e.g., a cavity or dental preparation in a tooth. Yet after the resin is polymerized it is very hard and durable and bonds well to the surrounding tissue.
One disadvantage of conventional resins is that they typically undergo shrinkage when they polymerize (i.e., harden). During polymerization, the polymeric monomers are chemically bonded, thereby becoming more ordered and capable of packing more closely together. A more ordered configuration generally decreases the volume of the polymeric material. The volume reduction experienced during polymerization is known as “polymerization shrinkage”.
Many attempts have been made to reduce or eliminate polymerization shrinkage. One common technique involves adding fillers to the composition. Because the filler does not undergo polymerization, the filler does not contribute to shrinkage. Thus, as the percentage of polymer in the composition is reduced, the overall shrinkage of the composition is proportionately reduced. Although adding fillers can reduce the amount of shrinkage, fillers alone cannot eliminate shrinkage because some amount of polymer will always be required. Furthermore, the amount of filler that can be included may be limited by required or desired performance properties in the final composite.
Others have attempted to counteract polymerization shrinkage by using particular monomers that undergo little or no shrinkage, or even experience expansion, during polymerization. For example, some prior art resins use cyclic monomers to carryout polymerization. In this case, ring opening increases the volume of the resin and offsets the shrinkage that occurs in the polymerization step.
While these compounds can reduce shrinkage, their use is very limited. First, the final properties of the composite depend on the monomers used to create the polymeric material. Therefore, when a particular monomer is used for its non-shrinking characteristics, it will also impart inherent characteristics in the final product. Moreover, because of the limited number of non-shrinking polymers that are available, the range of possible characteristics and properties are likewise limited. This technique of reducing polymerization shrinkage cannot be used with the many existing resins that have been developed to perform under specific conditions and for specific purposes. Furthermore, it would require coincidental circumstances to find a polymer for which polymerization shrinkage could be optimized that would result in optimal material properties.
While some techniques, such as using a filler or utilizing low shrinkage polymers, have minimized polymerization shrinkage, there exists a great need to further reduce polymerization shrinkage or completely offset its effects. Even small amounts of shrinkage can negatively affect resin performance. This is particularly true for certain applications such as dental sealers and fillers.
For example, following an endodontic root canal procedure, in which the root canal is cleaned using special root canal tools and irrigation devices, it is important to fill and seal the evacuated root canal to preserve the dead tooth from further decay that might compromise the integrity of the tooth and cause infection. In a typical procedure, one or more soft, resilient, needle-like inserts known as “gutta percha” points are inserted in each root canal branch in order at least partially seal and fill the root canal.
Conventional techniques require multiple gutta percha cones per canal and laborious “later condensation” techniques. For some, it requires heating the gutta percha in an attempt to make it flow into the lateral canals. However, this technique, coupled with the generally hydrophilic properties of gutta percha, make it hard to achieve fine adaptation to canal walls and flow into the dentinal tubules.
Resins in conjunction with gutta percha and antiseptic pastes have been used to fill and seal root canals following a root canal procedure. The composite resins provide a beneficial technique for filling and sealing a root canal because the resin can be inserted into the root canal in a fluid form and then hardened by polymerization in a curing step. The resins can also be designed to have a desired hardness and adhesiveness to the root canal wall to create a good seal.
Unfortunately, polymerization shrinkage has the potential of disrupting bonding between the sealer and the root canal wall, tooth, or other material. As the resin undergoes shrinkage during polymerization, the composite can pull away from the tooth material and break the seal or weaken the bond between the two. Even small gaps created in this manner can be problematic since bacteria can enter and cause decay or infestation.
Therefore, what is needed are dental filling and sealing compositions that are able to at least partially offset or eliminate the effects of polymerization shrinkage.